Cryptography Reference
In-Depth Information
decrypted. Synchronization errors, i.e., missing bits or blocks, are harder to
deal with. In fact, only the CFB mode can handle these errors. It is presumably
better to have a transmission protocol that excludes such errors from the outset,
rather than leaving the consequences up to the ciphering mode.
Another problem relates to
parallelism
. It has enormous practical significance:
network connections are getting increasingly faster, and encryption turns into a
bottleneck during data transmission. Of the modes introduced above, only ECB
supports the encryption of several blocks in parallel, i.e., it lets you operate
several ciphering units in parallel.
There is an
interleaved CBC mode
that also allows you to run several encryp-
tions in parallel. To this end, we decompose the data stream into about five
smaller streams, for example, arranged by block numbers as follows:
1st data stream:
1,6,11,16,...
2nd data stream:
2,7,12,17,...
...
5th data stream:
5,10,15,20,...
We take each data stream and encrypt it with its own chip in CBC mode.
There is a large number of other ciphering modes, in addition to the ones men-
tioned above. For example, the security software Kerberos 4 uses a mode called
PCBC
(
Propagating Cipher Block Chaining
), where the plaintext block,
P
n
+
1
, is XORed not only with
C
n
, but also with
P
n
. This mode guarantees
text integrity, which is important because an error in parallelism propagates
through all subsequent blocks. However, this feature is ineffective when two
successive blocks are swapped. Due to this theoretical weakness, PCBC is no
longer used in Kerberos 5 — details of the other modes are found in [SchnCr]
and [NISTmod].
Another unanswered question is the
cryptanalysis
of these methods. It is easy
to understand that one cannot generally decide whether the use of CBC, for
example, would make an encryption method more secure or more insecure.
The latter is unlikely to be the case with modern algorithms. The purpose of
ciphering modes is not to improve an algorithm, but to make its use more sturdy
or possible at all: ciphering errors like the repeated sending of the same plaintext
become ineffective thanks to careful CBC implementation; single bytes can
be transmitted in encrypted form in CFB mode, although one uses a block
algorithm.
